1. Field of the Invention
This invention relates to hydrodynamic bearings and particularly to a system of bearings featuring design and control of fluid flow in the bearing that optimizes stiffness and anti-vibration characteristics particularly applicable but not limited to airbearings.
2. Prior Art and Information Disclosure Statement
Bearings using air as a fluid have been designed for a variety of applications where it is required to have very low "drag" (minimal friction), where high speed is involved that would otherwise cause a rise in temperature, or where it is required to maintain close precision. The typical design of an airbearing is one in which pressurized air is admitted through an orifice in the stator opening into the interior of the airbearing interface; the air then escapes through exits at the edges of the interface, after following a flow pattern established by grooves in the stator surfaces.
One type of airbearing is the journal airbearing which comprises a rotating member supported by a static member. The interface between the rotor and stator is pressurized to airbearing support of the rotor. Another type of airbearing is the slider bearing in which a yoke straddles a bar and the interface between the yoke and bar is pressurized to provide airbearing support of the slider.
Both the journal and slider bearings rely on hydrodynamic pressure in the interface generated by flow of air through the interface. By hydrodynamic pressure is meant the pressure that is generated by virtue of the viscous nature of the lubricant. It will therefore be understood that the scope of this invention includes liquid and gaseous lubricants since both types of lubricants are characterized by a coefficient of viscosity.
In the context of this specification, the term hydrodynamic bearing may be taken to mean a bearing lubricated by either liquid or gas.
Two major problems characterize the journal and slider airbearings:
One problem is that such bearings will tolerate very little asymmetric force (sidewise, unidirectional force) compared to a hydrodynamic bearing utilizing a liquid lubricant. Such asymmetric forces may result in collapse of journal and slider airbearings. A journal bearing will support only a relatively small radial load. A slider airbearing will support only a relatively small force in the direction transverse to the direction of sliding motion. Therefore the conventional journal airbearing is designed to have an airflow pattern that is very symmetrical about the axis of rotation. The slider airbearing is designed to have an airflow pattern that is very symmetrical between both sides of a centerline parallel to the direction of motion.
U.S. Pat. No. 4,930,907 to Smith discloses construction of a journal airbearing in which a first airbearing area is disposed on one side of the rotor at the same angular position as an applied radial load and a second airbearing area is disposed on the opposite side. The location and sizes of these areas and the single value of air pressure applied to both bearing surfaces are all selected to balance the total force and moment on the bearing, including the load applied to the bearing, thereby establishing mechanical equilibrium. However, application of this design is limited to situations where the axial location of the applied radial load is fixed. Furthermore, there is no provision for optimizing the pressure to individual bearings to achieve maximum stiffness.
U.S. Pat. No. 4,930,907 to Smith discloses several devices for sensing applied load in airbearing systems. One construction is a mechanical approach in which pressure applied at the load is also applied at the airbearings. Another approach is electronic in which an electronic signal responsive to an applied load controls pressure differential between bearing interfaces. A third approach is hydraulic, in which hydraulic pressure signals generated by an applied load control the pressure differential between bearing interfaces.
U.S. Pat. No. 3,772,296 is for an airbearing system using a self-regulating regulator. The bearing system includes a regulator communicating with a "gauging" port located at the bearing interface which transmits pressure to the regulator which responds to the pressure signals to restore a preset pressure in the interface. The system does not address the problem of optimizing pressure for maximum stiffness.
Another problem with airbearings is that, under some conditions of applied air pressure and design, the bearing will resonate. The standard approach to eliminating resonance has been to design the bearing with certain parameters such as size of the orifice, area of land around the orifice, etc. However, onset of resonance has been very difficult to predict and these design approaches have been met with only limited success. Resonance normally occurs when the air pressure is raised to a critical value.
For further discussion of airbearing technology, the reader is referred to "Gas Lubricated Bearings" by Grassam and Powell, published by Butterworth, London, 1964 which is incorporated by reference into this specification.